Devices with at least one oscillation element, so-called vibration detectors, have been known for detecting or monitoring the level of a medium in a container. The oscillation element is usually at least one oscillation bar, which is secured to a membrane. The membrane is caused to oscillate using an electro-mechanical transducer, e.g. a piezo-electric element. Due to the oscillations of the membrane, the oscillation element secured to the membrane also oscillates.
Vibration detectors embodied as level detectors utilize the effect that the oscillation frequency and the oscillation amplitude depend on the particular degree to which the oscillation element is covered: While the oscillation element can carry out its oscillations in air free and undamped, it experiences a damping, and, as a consequence thereof, a frequency and amplitude change, as soon as it is submersed partially or completely in the medium. On the basis of a precalibrated frequency change, an accurate conclusion can be drawn as to the particular level in the container. Level measuring devices are also applied especially for overfill protection or as protection against pumps running empty.
The oscillation frequency of the oscillation element is also influenced by the particular density of the medium. Consequently, at constant degree of covering, there is a functional relationship for the density of the medium, so that vibration detectors are best suited both for level determination and for density determination. In practice for monitoring and detecting the level or density of the medium in the container the oscillations of the membrane are picked up and changed into electrical received signals by means of at least one piezo-element.
The electrical received signals are then evaluated in an evaluation electronics. In the case of a level determination, the evaluation electronics monitors the oscillation frequency and/or the oscillation amplitude of the oscillation element and signals the condition ‘sensor covered’ or ‘sensor uncovered’, as soon as the measured values move under, or over, a pre-assigned reference value. A corresponding report to the operating personnel can then be given optically or acoustically. Alternatively or supplementally, a switching process is triggered; in this way perhaps a feed or drain valve on the container is opened or closed.
The devices mentioned above for measuring level or density are used in a multitude of industrial branches, for instance in chemistry, in the foods industry or for water treatment. The band width of monitored fill materials reaches from water through yogurt, paints and lacquers, to highly viscous fill materials, like honey, or to strongly foaming fill materials, like e.g. beer.
Vibration detectors are, however, only completely dependent on the above-mentioned parameters ‘level’ and ‘density’ to a first approximation. Besides these, other physical parameters also influence the oscillation behavior of the oscillation element, process parameters such as pressure and temperature or the viscosity of the medium. Thus, as soon as the requirement is made, that the sensor be applied for highly accurate measurements or that it be used as a universally applicable measuring device in the high and low temperature range or in the high or low pressure range, then the influence of these parameters on the oscillation behavior must be taken into account. In principle, the influence of temperature and pressure on the measurement results becomes that much more important, the greater these parameters deviate from normal conditions. Similar considerations are true also with respect to the viscosity of the medium: A measurement device must in the future be able to produce reliable measurements for media of greatly differing viscosities.